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Water hammer

I'll start by saying I'm not a plumber and I do very little plumbing. I do lots of hydronic heat, HVAC and ref work, but try to stay away from plumbing, I need some help with water hammer.

Replaced a tank for a friend (of course). Removed a natural draft tank located in centre of house, and installed powervented tank 6' away on outside wall. Changed no other plumbing, 3/4 in and out of tank.

Before install, no water hammer (I know this as I lived there a while). After install crazy bad water hammer on ALL fixtures, hot AND cold, even when the toilet shuts off! Tried installing 2 of those little Oatey water hammer arresters in both the hot and cold mains, as high as possible and close to tank, did little to suppress the problem.

With tank valves shut off, no water hammer at all. With air pockets in the lines, the hammer goes away, but once all the fixtures are are operated, and the air is gone, it's back.

I know I could put a bigger shock arrester in and likely stop the noise, but I really want to FIX the problem

I'm at a loss, any suggestions?

The tank is not %100 level, but its fairly close, could that effect it?